Hydrazinium nitroformate propellant stabilized with nitroguanidine

ABSTRACT

The stability of solid propellant compositions containing unsaturated carboxyl terminated hydrocarbon binders and hydrazinium nitroformate as an oxidizer will have improved shelf life and stability through the incorporation of nitroquanidine in the mixture.

United States Patent Low et al. [4 1 Apr. 25, 1972 1541 HYDRAZINIUMNITROFORMATE [56] References Cited PROPELLANT STABILIZED WITH UNITEDSTATES PATENTS NITROGUANIDINE 3,046,829 7/1962 Roemer ..l49/I9 X [72]Inventors: George M. Low, Deputy Admlmstrator of 3,147,161 9 9 Abel-e et#14956 x the National Aeronautics and Space Ad- 3 305 523 2 19 7Burnside 9 19 X ministration with respect I9 an invention 3,418,18312/1968 Rice ...149/19 x Vernon y, 5059 Alta Street, 3,476,622 11/1969Harada eta]. ..149/19 Santa Susan, Calif. 93063 22 t. 1 7 PrimaryExaminer-Carl D. Quarforth 1 sep 9 0 Assistant Examiner-15. A. Miller[2]] Appl. No.: 74,861 Attorney-Monte F. Mott,J.H.Warden and John R.Manning [52] US. Cl ..149/19, 149/20, 149/36, [57] ABSTRACT 149/92 Thestability of solid propellant compositions containing un- [5l] lnt.Cl...C06d 5/06 saturated carboxyl terminated hydrocarbon binders and [58]Field of Search ..l49/l9, 36, 92, 20, 88 hydrazinium nitroformate as anoxidizer will have improved shelf life and stability through theincorporation of nitroquanidine in the mixture.

9 Claims, No Drawings HYDRAZINIU M NITROFORMATE PROPELLANT STABILIZEDWITH NITROGUANIDINE ORIGIN or THE INVENTION The invention describedherein was made in the performance of work under a NASA contract and issubject to the provisions of Section 305 of the National Aeronautics andSpace Actof 1958, Public Law 83-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1 Field of the Invention This invention isin the field of solid propellant compositions. More particularly, theinvention relates to improved solid propellant composition containinghydrazinium nitroformate and unsaturated carboxyl terminated hydrocarbonbinder.

2. Description of the Prior Art Hydrazinium nitroformate, HNF, is a verydesirable oxidizer for use in solid propellant formulations because itis very energetic, thus providing high performance. One of the mostpopular binders utilized in solid propellants is carboxyl terminatedpolybutadiene. This binder has a plurality of double bonds in thebackbone. It has been found that HNF cannot be satisfactorily utilizedwith binders containing such double bonds, since it appears that HNFattacks the backbone of the binder, breaking down the binder chain.Pockets of gas are formed andthe propellant thus swells. Further, due tothe breakdown of the binder backbone, the material becomes soft. Thus,where a binder containing double bonds is utilized, a typical shelf lifewith HNF will range from 2 to days at a temperature of 70-90 F. Such ashort shelf life prevents practical utilization of HNF with binderscontaining double bonds.

An additional problem in utilizing HNF in solidpropellants isthatit willattackmany of the typical curing agents utilized. Particularly it isfound that the HNF will attack the aziridine type of curing agent, suchas MAPO, which is tris[1-(2- methyU-aziridinyl] phosphine oxide. One canovercome the problem of the'HNF attack on the curing agent by utilizinga curing agent such as triethyelene melamine which is not susceptible toreaction with the HNF. However, it is not possi ble to readily selectamore suitable binder than the carboxyl terminatedpolybutadiene. As aresult, in order to have a practical system, additionalmeansisrequiredto prevent reaction between theHN F andthe unsaturated hydrocarbonbinder.

SUMMARY OF THE INVENTION The herein invention is based upon thediscovery that if a relatively small amount of nitroguanidine is addedto solid propellant formulations having unsaturated hydrocarbon bindersand containing HNF, then the shelf life of the propellant issignificantly increased. Thus, unexpectedly it has been discovered thatif at least 2 weight percent of the nitroguanidine is utilized, theshelf life of the propellant containing HNF canbe increased up to atleast 5 months at ambient temperatures. The nitroguanidine is added tothe propellant during its mixing phase and will remain in thecomposition to prevent the undesirable reaction of the HNF with thebinder. It is believed the invention will be further understood from thefollowing detailed description and example:

DESCRIPTION OF THE PREFERRED EMBODIMENTS As has been pointed out, HNF,though being a very energetic oxidizer, appears to attackthe'doublebonds that exist in various hydrocarbon binders utilized insolid propellants. The result of this attack is the production of a gaswhich swells the propellant, making it spongy or porous. The material isthen no longer utilizable for rocket motors, since it will havecompletely unpredictable burning rates. Further, the strength of thepropellant is so greatly weakened that it cannot sustain the highg-loads encounteredin rocket motors without further disintegrating. Themost typical binder that is utilized with HNF is carboxyl terminatedpolybutadiene. Another binder which is useful, but not as preferred ispolyisoprene, which also contains double bonds susceptible to attack bythe HNF. There are other solid oxidizers known to attack solidunsaturated binders, such as hydroxylamine perchlorate and hydrazineperchlorate. These other oxidizers however are not as energetic as HNFand thus the herein invention is not directed toward preventing theattack of these materials upon the unsaturated binders. These othermaterials are merely illustrative of the existence of the herein problemin solid propellant systems.

A typical solid propellant formulation utilizing HNF as a solidparticulate oxidizer will contain on the order of 50 to 65 weightpercent. of the material. The unsaturated hydrocarbon binder willcomprise from 5 to 25 weight percent. Additionally, there will be asolid particulate fuel such as aluminum, ranging from 5 to 20 weightpercent. Propellant compositions will preferably additionally containother materials, such as plasticizers, including hydrocarbon oils,certain esters and some nitro compounds which can range from 0 to 20weight percent. Representative of hydrocarbon oils are Conoco H-35, aproduct of Continental Oil Co. and Oronite Polybutene No. 6, a productof California Chemical Co. Esters, in order to be miscible with thehydrocarbon polymer binder, must have a carbon to oxygen ratio of about3 to l or higher. Dioctyl azelate and isodecyl pelargonate are typicalof the preferred esters.

Nitro and nitrato compounds also are not soluble to a great extent inthe polybutadiene binder when the carbon to oxygen ratio is less thanabout 3 to 1. Thus, the low solubility of triethylene glycol dinitrateand trimethylol ethane trinitrate where the c/o ratio is less than 1 tol limits their use to less than 5 parts per parts of the polymer.

Further, a small amount varying from 0.2 to 1.0 weight percent of thecuring agent, preferably triethylene melamine, is present to cross linkthe unsaturated hydrocarbon binder. It is particularly preferred to usethe melamine as a curing agent since it neither attacks or reacts withHNF, unlike other various conventional curing agents that are used withthe unsaturatedbinders.

Without further additive materials, the above general compositionof asolidpropellant containing HNF will decompose within a relatively shorttime at ambient conditions. However, upon the addition of nitroguanidinetothe'formulation, shelf life can be increased to the point where thepropellant is use ful. It has been found that at least 2 weight percentof nitroguanidine is required in the propellant formulation. This amountcan vary up to 20 weight percent of the formulation, at which point nofurther improvement in propellant stability is seen upon furtheraddition of the nitroguanidine. Generally it is preferred to add about10 weight percent of the nitroguanidine to assure inhibition of gasformation due to the aforegoing reaction of the HNF.

Nitroguanidine has been previously added to various solid propellantcompositions mainly to serve as a means to control the burning rate ofthe propellant. In the herein compositions, the nitroguanidine howeverperforms unexpected results, since other similar materials which havebeen tried do not affect the reaction of HNF with the binder at all orto a minor degree as compared to the nitroguanidine. For example, twonitrocellulose stabilizers, ethyl contralite and 2-nitrodiphenylamine,when added to the composition herein in fact increased the rate ofdeterioration of the HNF containing propellants. Another material thathad been added was resorcinol, which in some tests appeared to slightlyincrease the shelf life but was not nearly as effective as thenitroguanidine. Various other materials that had also been tested had noeffect, or in fact accelerated the deterioration. Thus, unexpectedlynitroguanidine inhibited the reaction of the HNF with the binder toprovide a useful propellant composition.

The nitroguanidine is obtained in a powder form and can be added at anytime during the mixing of the propellant ingredients. In the mixing ofthe propellant, normally the binder and plasticizer are first mixed withthe fuel then the oxidizer is added thereto. The nitroguanidine is oftenadded with the fuel.

It is believed the invention will be further understood from thefollowing detailed examples.

EXAMPLE 1 A propellant formulation was made by mixing in weight percentl0.92 percent of carboxy terminated polybutadiene, 0.34 weight percenttriethylene melamine, 4.74 percent Conoco l-l-35, which is a hydrocarbonoil with a low viscosity, 16 percent aluminum powder, and 68 percentHNF. The carboxy terminated polybutadiene, plasticizer and aluminum fuelwere mixed for five minutes in a Baker-Perkins vertical mixer. The HNFwas added thereto in three increments, mixing for 10 minutes after eachaddition. Finally, the curing agent was added and mixing was continuedfor 10 minutes in vacuum to obtain a void-free mix. This propellantcured in about 24 hours at ambient temperature. Swelling and porosity ofthe propellant mixture became evident after 6 days at ambienttemperature.

EXAMPLE ll A composition containing 13.9 weight percent carboxylterminated polybutadiene, 0.6 weight percent triethylene melamine, 3.5weight percent Conoco H-35, l percent aluminum powder and 62 percent HNFwas formulated as described in Example I. To this composition during themixing thereof was added 10 percent nitroguanidine. The propellant wascast into plastic containers which had a length to diameter ratio of 3.The propellant cured in about 24 hours at ambient temperature then wasstored at ambient temperature and was examined frequently.

The above formulation of this example showed no deterioration such asswelling or softening after 5 months storage at ambient temperatures.

EXAMPLE [I] To indicate that nitroguanidine appeared to particularlyachieve the unexpected results of the herein invention a relatedcompound, cyanoguanidine, was utilized as an additive to the propellantformulation. A composition was thus formulated containing 12.4 percentcarboxy terminated polybutadiene, 0.5 percent triethylene melamine, 3.1percent Conoco H-25, 10 percent aluminum powder, 68 percent HNF, and 6percent cyanoguanidine. The resulting cured composition began swelling13 days after mixing at ambient temperature.

Thus, as can be seen from the examples and above description,nitroguanidine unexpectedly and peculiarly serves to inhibit a reactionbetween HNF and unsaturated hydrocarbon binders, particularly carboxylterminated polybutadiene. No theoretical explanation for the effect ofnitroguanidine has been developed. The possibility that the stabilizingeffect might be due to the nitramino group (--NHNO was considered.Ethylene dinitramine, containing two of these groups. however, was noteffective.

Iclaim:

l. A solid propellant composition comprising:

an unsaturated carboxyl terminated polymeric hydrocarbon binder,

hydrazinium nitroformate is an amount sufficient to act as an oxidizer,

a suitable curing agent for said binder,

and nitroguanidine as an amount sufficient to improve the shelf life ofthe composition.

2. The composition of claim 1 wherein said binder is carboxyl terminatedpolybutadiene.

3. The composition of claim 1 wherein said curing agent is triethylenemelamine.

4. The composition of claim 1 further comprising:

a solid particulate metal fuel.

5. A solid propellant corn osition co'm rising:

5 to 25 weight percent of unsaturate carboxyl terminated polymerichydrocarbon binder,

50 to 65 weight percent hydrazinium nitroformate,

0.2 to 1.0 weight percent ofa curing agent for said binder,

and 2 to 20 weight percent nitroguanidine.

6. The composition of claim 5 further comprising:

5 to 20 weight percent ofa solid particulate metal fuel.

7. The composition of claim 6 wherein said binder is carboxyl terminatedpolybutadiene.

8. The composition of claim 7 wherein said curing agent is triethylenemelamine.

9. The composition of claim 5 additionally comprising:

up to 20 weight percent of a suitable plasticizer for said binder.

2. The composition of claim 1 wherein said binder is carboxyl terminatedpolybutadiene.
 3. The composition of claim 1 wherein said curing agentis triethylene melamine.
 4. The composition of claim 1 furthercomprising: a solid particulate metal fuel.
 5. A solid propellantcomposition comprising: 5 to 25 weight percent of unsaturated carboxylterminated polymeric hydrocarbon binder, 50 to 65 weight percenthydrazinium nitroformate, 0.2 to 1.0 weight percent of a curing agentfor said binder, and 2 to 20 weight percent nitroguanidine.
 6. Thecomposition of claim 5 further comprising: 5 to 20 weight percent of asolid particulate metal fuel.
 7. The composition of claim 6 wherein saidbinder is carboxyl terminated polybutadiene.
 8. The composition of claim7 wherein said curing agent is triethylene melamine.
 9. The compositionof claim 5 additionally comprising: up to 20 weight percent of asuitable plasticizer for said binder.